Search

A detailed study, based on a coupled mineralogical and geochemical approach, was done on IODP Site U1512 sediments (Bight Basin, Southern Australia). The mineral assemblages of the clay size fraction (<2 μm), obtained by X-ray diffraction, have been compared to inorganic (major and trace elements) geochemical signals, obtained from both whole rock and clay size fraction, and Sr/Nd isotopes, obtained from clay size fraction to determine climatic and weathering conditions during the Turonian to Santonian interval (∼94–84 Ma, Late Cretaceous) in the southern high latitude band. This study reveals that the mineral assemblages of clay fraction consist of R0-type illite/smectite mixed-layers (considered as smectites), illites, and kaolinites associated with opal-CT and clinoptilolites. Electron microscopy observations have demonstrated a negligible impact of burial diagenesis on clay assemblages while authigenesis contributions of clay minerals are limited and associated with minor volcanic contributions. Clay assemblages would be thus dominantly primary in origin. Based on Sr and Nd isotopic values, clay minerals are interpreted as the resulting from the weathering of rocks and pedogenic blankets of Eastern Australian Highlands and from the remobilization of older sediments from nearby uplifted basins. The chemical compositions of both whole rock and clay size fraction show a relative stability during the studied interval with two exceptions: during the Middle/Late Turonian times for Si, P and Ba and at the Turonian-Coniacian boundary for Ti, K and Zr, associated respectively to an increase of biogenic fraction and to an enhanced input of terrigenous material. The clay assemblages, dominated by smectites, show weak variations along the borehole reflecting stable climatic conditions during the 10-Myr Turonian-Santonian interval. The dominance of smectites, coupled to the absence of chlorite and the low proportions of illite, which are both considered as primary clay minerals, indicates a dominant warm/temperate and sub-humid climate during the studied interval. However, changes in weathering conditions occurred during the Turonian-Santonian interval in southern high latitude. The Early Turonian, characterized by high Chemical Index of Alteration values (>75), is followed by an interval of lower chemical weathering with enhanced biogenic silica during the Middle/Late Turonian. The slight increase of kaolinites, recorded during Coniacian and coeval with increase of Ti/Al and Zr/Al ratios, probably reflects an increase in coarser terrigenous inputs associated with a sea-level fall. • Clay mineral assemblages record stable weathering conditions in southern high latitudes during the Turonian-Santonian interval. • A high contribution of biogenic silica is recorded in site U1512 during the Middle/Late Turonian. • Slight varations in kaolinite proportions are associated to sea-level fall. [ABSTRACT FROM AUTHOR]

Jabel Sayed, in the northern part of the Arabian Shield, is an alkaline granite complex. It consists of syenogranites and aegirine–riebeckite alkali feldspar granites intruded into an older granodiorite–tonalite suite. Elemental geochemistry attests that the syenogranite and the alkali feldspar granite represent a genetically related post-collisional A 2 -type granite intrusion (e.g., high K 2 O + Na 2 O, FeO ∗ /MgO, Ga/Al, and Y/Nb, and low CaO, Ba, and Sr). U–Pb zircon dating and Nd isotopes of the alkali feldspar granites indicate an age of 593 Ma, εNd( t ) values of +4.9 to +5.3 (mean = +5.1), and depleted mantle model ages ( t DM ) between 0.78 and 0.88 Ga. These isotope results are in agreement with a juvenile magma source of Neoproterozoic age like that responsible for forming most other Arabian–Nubian Shield crust. The alkali feldspar granites have substantial modal content of albite and fluorite, wide variation of HFSE and REE and exhibit tetrad effects in REE patterns that indicate interaction with hydrothermal F-rich fluids. The fluids caused secondary metasomatic alteration superimposed on the granitic rocks. The result of the fluids–granitic rock interaction is a metasomatic pegmatite rim, which carries economic levels of HFS elements and REE, and a red granite variety (altered syenogranite) characterized by large variation in trace element abundances. Geochemical features in the granitic rocks such as strong linear positive correlation of Ta versus Nb and Zr versus Hf indicate magmatic origin of the fluids and emphasize that the behavior and enrichment of Ta and Nb are largely controlled by magmatic processes. Based on the geochemical and Nd isotopes compositions, we propose an origin for the Jabel Sayed granites which involved fractional crystallization of a parental magma derived by the partial melting of a crustal protolith accreted between 780 Ma and 880 Ma. This crustal protolith has juvenile geochemical and isotopic composition and was extracted from a subduction modified mantle source. [ABSTRACT FROM AUTHOR]